java如何获取"当天或指定日期"是周几呢?

实现思路:
    1.用Calendar#DAY_OF_WEEK 来获取
    2.用SimpleDateFormat 的 u 周标识,格式化日期来获取
    3.用LocalDate.getDayOfWeek() 来获取

package com.java265;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.ZoneId;
import java.time.format.DateTimeFormatter;
import java.time.temporal.ChronoField;
import java.util.Calendar;
import java.util.Date;
import java.util.TimeZone;

/**
 * 获得指定是周几的方法
 */
public class DateUtils {

    public static void main(String[] args) throws ParseException {
        getWeekFromStrDateByCalendar("2022-09-06");
        getWeekFormStrDateByFormat("2022-09-06");
        getWeekFromStrDateByLocalDate("2022-09-06");
    }


    static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
    static DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");

    /**
     * @see java.util.Calendar#DAY_OF_WEEK
     * @see java.util.Calendar#SUNDAY
     * Calendar 对象中 周日开始的 标识 1, 周六 标识 7
     * 将对象转为 Calendar 对象,在用该对象的 get 方法,带入指定字段获取 对应的 值 (DAY_OF_WEEK)
     * @param str
     * @throws ParseException
     */
    public static void getWeekFromStrDateByCalendar(String str) throws ParseException {
        Date parse = simpleDateFormat.parse(str);
        Calendar instance = Calendar.getInstance(TimeZone.getTimeZone(ZoneId.of("Asia/Shanghai")));
        instance.setTime(parse);
        int i = instance.get(Calendar.DAY_OF_WEEK);
        System.out.println(i);
    }

    /**
     * 根据 SimpleDateFormat 对象的 说明,提示,发送 周几 对应的格式符 为 'u'
     * @param str
     * @throws ParseException
     */
    public static void getWeekFormStrDateByFormat(String str) throws ParseException {
        Date parse = simpleDateFormat.parse(str);
        // u 指定周几
        SimpleDateFormat format = new SimpleDateFormat("u");
        String format1 = format.format(parse);
        System.out.println(format1);
    } 

    /**
     * @param str
     * @throws ParseException
     */
    public static void getWeekFromStrDateByLocalDate(String str) throws ParseException {
        LocalDate parse = LocalDate.parse(str, formatter);
        DayOfWeek dayOfWeek = parse.getDayOfWeek();
        System.out.println(dayOfWeek.toString());
        System.out.println(dayOfWeek.get(ChronoField.DAY_OF_WEEK));
    }

}